Positive displacement, oval rotor type flow meter designs are well known and have been employed successfully in a number of industries including the petrochemical, power, gas, food, and beverage industries. Their popularity is due, in part, to the fact that their measurement accuracy does not depend on intricately shaped cams which can become misaligned, as is the case with sliding vane type positive displacement flow meters. Furthermore, unlike nutating disc type flow meters, the calibration factor does not vary with the viscosity of the fluid being metered.
In the case of oval type flow rate meters, a pair of oval shaped gear toothed rotors are used to sweep out a precisely known volume of the fluid passing through a measurement chamber during each rotor rotation cycle. In general, the rotors are positioned relative to one another in the chamber such that the gear teeth disposed on their surfaces mesh together at the point of articulation, and remain as such during the entire rotation cycle. It should be noted, however, that ideally none of the fluid being metered actually passes directly between the gear teeth themselves. The intermeshed nature of the gear teeth, in combination with the inherent viscosity of the fluid being metered, is intended to prevent the flow of fluid between the two articulating rotor surfaces. Instead, the metered fluid is swept out in the precisely measured pocket created by the oval shape of the rotor and the inner chamber wall as the rotors rotate about their respective shafts.
In general, as the viscosity of the metered fluid increases the accuracy obtainable by the flow meter also increases, due to a reduction in the ability of the fluid to seep between and through the intermeshed gear teeth of the two articulating rotors. However, in practice, even with higher viscosity fluids, a certain amount of fluid tends to become trapped or squeezed between the gear teeth of the intermeshed rotors during the course of a normal rotor rotation cycle. As a result, typical geared oval rotors must have cuts or grooves formed in the gear teeth to allow this trapped fluid to escape. Unfortunately, such relief cuts or grooves in the teeth necessarily create leakage or unmeasured flow paths between the intermeshed rotors and consequently results in flow measurement inaccuracies.
Therefore, there remains a need for a practical and economical oval rotor type, positive displacement flow meter which efficiently meters the fluid flowing therethrough, while minimizing leakage or unmeasured flow between the articulating rotor surfaces.